Nonwoven fibrous mat for MERV filter and method of making

ABSTRACT

Wet laid, nonwoven, fibrous mats suitable for making MERV 6 or higher filters, and methods of making the nonwoven mats are disclosed. The mats include a blend comprising a major portion of glass fibers and a minor portion of man-made polymer fibers, such as polyester fibers, the fibers being bound together with a cured binder having a Tg in the range of about minus 30 to about 50 degrees C. The mats are pleated prior to assembling the filter in conventional manner. The only fibrous mat in the filters is wet laid fibrous nonwoven mat and the pleated filter media needs no other means of support in the filter frame.

The present invention relates to methods of making fibrous, nonwovenmats for use in air filter fabrication and other applications wheresimilar requirements exist and the mats so made.

BACKGROUND

A relatively new ASHRAE standard abbreviated MERV shows a filter'sminimum performance through its life, allowing the contractor orbuilding owner to select filters knowing their “worst case” efficiency.It measures a filter's ability to remove particles of specific sizeswhen the filter is new. The old standard does not tell you a filter'sefficiency in removing specific particle sizes (such as lung-damagingrespirable particles). By comparison, with the ASHRAE 52.2-1999 test,particle counters measure the number of airborne particles withdiameters of 0.3 to 10.0 microns, both upstream and downstream of theair filter. Using this information, it becomes possible to take a highlytargeted approach to filter selection. Once the test is completed, thefilter's minimum efficiency values at various particle sizes arerecorded. These efficiency values are then used to assign a MERV numberto the filter. Designations range from MERV 1 (typically a lowefficiency, throwaway filter) up to MERV 16 (a 95%-plus ASHRAE filter).The new MERV system is much more comprehensive than previous systems,and it enables one to compare efficiencies of filters at a glance.Minimum Efficiency Reporting Value, or MERV for short, is a filterrating system devised by the American Society of Heating, Refrigerationand Air conditioning Engineers (ASHRAE) to standardize and simplifyfilter efficiency ratings for the public. The higher the MERV rating,the higher the efficiency of the air filter. Simply stated, a MERV 12filter will remove smaller particles from the air. than a MERV 8 filter.

The MERV system allows the consumer to effectively compare one brand offilter to another. Without any value-added additions, any MERV 6 filterwill perform about the same as any other MERV 6 filter. The MERV ratingonly applies to efficiency of the filter. The presence of otherfunctional ingredients in the filter, such as anti-bacterial treatmentand/or baking soda, are value-added benefits and are not a part of theMERV rating system. In the 3-10 micron range of particle diameters, aMERV 6 filter will be about 35-50 percent efficient. A particle that is10 microns or less in size is not visible to the naked human eye. Ifallergies or asthma are your concern, we suggest you choose a MERV 8filter at a minimum.

In the past furnace filters were made from relatively coarse continuousfibers laid down in a random pattern and built up in layers on a drumand by carding dry staple fibers and forming webs of the carded fibers.Such filters, in recent years at least, fall short of removing as smallof particles and as many particles as desired. Average efficiency isreally not a realistic measure of filter performance because itexaggerates performance for the early part of the filter's actualservice life. This is because when an air filter is first installed itsefficiency is at its lowest point because it hasn't built up enough lintand particles on the filter to help trap more and smaller lint andparticles. Some of these filters had low efficiencies and others hadother disadvantages such as low physical integrity and high bulkpreventing pleating or making it very difficult. Also, cost is an issue.In U.S. Pat. No. 6,579,350 some of this is confirmed and addressed byusing a layer of wet laid fibers to attempt to minimize the shortcomingsof dry or air laid filtration media, but that process and filter isstill complex and expensive.

SUMMARY OF THE INVENTION

The present invention is a wet laid nonwoven mat for use as filtrationmedia, a filter made using this wet laid media and the method of makinga low cost, efficient gas filter, the filtration media and the filterhaving a MERV rating of at least 6. The filtration media of theinvention is a wet laid fibrous mat comprising about 60-20 wt. percentglass fibers, about 15-60 wt. percent polymer fibers, includingpolyester, and about 15-40 wt. percent, dry basis, of a binder to bondthe fibers together. The major portion of the glass fibers have adiameter of at least about 3 microns and up to about 16 microns with 11microns being particularly effective. The polymer fibers usually have adenier of about 1.5 and can range from microdenier up to about 6 denier.The binder is usually a latex such as an acrylic resin modified withmelamine formaldehyde having a glass transition temperature in the rangeof about minus 30 to about 50 degrees C. or other resins having a glasstransition temperature in this range. A more typical glass temperaturerange is from about 20 to about 50 degrees C. Binders with a glasstransition temperature in the 25-50 degree C. range yield a filterelement with self supporting pleats and eliminate the need for expensivewire or other complex supports. Additionally, cellulosic fibers or asmall percentage of fine glass fibers like glass microfibers can beadded to increase strength and filtration efficiency.

The method of the invention comprises;

-   -   a) dispersing fibers in a conventional white water containing a        viscosity enhancing agent to produce a dispersion, the        dispersion comprising glass fibers and man-made polymer fibers,    -   b) subjecting the dispersion to a moving permeable forming belt        to form a fibrous web,    -   c) applying an aqueous resin binder to the wet web and removing        any excess binder to produce the desired binder content in the        wet web, and    -   d) drying the wet web and curing the resin in the binder to form        a resin bound fibrous non woven filtration media    -   e) pleating the filtration media and    -   f) assembling the pleated filtration media into a filter for        filtering air and other gases, the improvement comprising:    -   dispersing glass fibers and polymer fibers in the conventional        whitewater in a ratio of about 0.4-3 parts glass fiber to 1 part        of polymer fibers, the glass fibers having a diameter of about 3        microns to about 16 microns and a length between about 0.2 inch        and about 1 inch, the polymer fibers having a denier in the        range of less than 1 to about 6 and a length in the range of        about 0.25 inch to about 0.5 inch, and applying an aqueous latex        binder having a glass transition temperature in the range of        about minus 30 to about 50 degrees C. in sufficient amount to        produce a dry binder content in the range of about 15 to about        40 wt. percent in the dry filtration media, whereby the        resultant filter media, when pleated and formed into a pleated        gaseous filter using conventional pleating and assembly        techniques produces a filter meeting at least MERV 6        specifications and requires no additional means of support in        the pleated gaseous filter.

The filtration efficiency can be controlled by varying the basis weight,the fiber diameter and type of fiber, and the basis weight of the wetlaid filtration media. The binder choice can also affect sheet densityby forming “bridge” or “webs” between the fibers at their crossingpoints thus reducing porosity of the mat. The filtration performance ofthe novel wet laid filtration media of the invention and resultantfilters is surprising to many in the industry because they have not beenexposed to this type of filtration media.

When the word “about” is used herein it is meant that the amount orcondition it modifies can vary some beyond that so long as theadvantages of the invention are realized. Practically, there is rarelythe time or resources available to very precisely determine the limitsof all the parameters of ones invention because to do would require aneffort far greater than can be justified at the time the invention isbeing developed to a commercial reality. The skilled artisan understandsthis and expects that the disclosed results of the invention mightextend, at least somewhat, beyond one or more of the limits disclosed.Later, having the benefit of the inventors disclosure and understandingthe inventive concept and embodiments disclosed including the best modeknown to the inventor, the inventor and others can, without inventiveeffort, explore beyond the limits disclosed to determine if theinvention is realized beyond those limits and, when embodiments arefound to be without any unexpected characteristics, those embodimentsare within the meaning of the term “about” as used herein. It is notdifficult for the artisan or others to determine whether such anembodiment is either as expected or, because of either a break in thecontinuity of results or one or more features that are significantlybetter than reported by the inventor, is surprising and thus anunobvious teaching leading to a further advance in the art.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The polymer fibers are preferably polyester fibers, but can also be anypolymer fiber such as polypropylene, nylon, PBT, polyacrynitrile,polybenzimidizole, and other known polymer fibers having similarresilience, toughness and a softening point high enough to tolerate thetemperatures used in the mat manufacturing process and subsequentprocesses that the mats are used in. The preferred diameter of thepolyester fibers is about 1.5 denier, but both the length and diametercan be varied so long as the aspect ratio, length to diameter, remainswithin a range suitable satisfactorily dispersing the fibers in anaqueous inorganic fiber slurry suitable for forming a web on an wet laidweb forming machine, such as an inclined wire former such as a VOITHHYDROFORMER® or a SANDY HILL DELTAFORMER®. The preferred length of 1.5denier polyester fibers is 0.25 inch.

The denier of the polyester fibers can range from about 0.8 to about 6denier and the fiber length will often be changed depending on thedenier to get good dispersion, as is well known. The man-made polymerfibers can, but need not be, longer as the denier is increased. Iftangling and/or roping causing clumps or bundles during dispersion, thelength of the man-made polymer fibers must be reduced to get gooddispersion.

The inorganic fibers are typically glass fibers and typically 0.5 inchlong, 11 micron diameter, E glass fibers having a chemical sizingthereon as is well known. One fiber product preferred for use in thepresent invention is H 117, a wet chopped fiber product available fromJohns Manville Corporation of Denver, Colo., but any type of glass fibercan be used in lengths and diameters suitable for the wet laidprocesses. Any type of stable glass fibers can be used, such as A, C, S,R, E and other types of glass fibers. Typically the average fiberdiameter of glass fibers will range from about 8 to about 20 micronswith fiber length ranging from about 0.25 to about 1.5 inches, moretypically from about 0.25 to about 1 or 1.25 inches and most typicallyfrom about 0.3 to about 0.75 inches.

The fiber blend webs are bound together by use of an aqueous bindercomposition applied with a curtain coater, dip and squeeze, roller coat,or other known saturating method in a known manner and the resultantsaturated wet bindered web laying on a supporting wire or screen is runover one or more vacuum boxes to remove enough binder to achieve thedesired binder content in the mat. The binder level in the inventivemats can range from about 10 to about 35 or 40 wt. percent of thefinished dry mat, preferably about 15 to about 30 wt. percent and mosttypically from about 18 to about 25 wt. percent, such as about 20±3 wt.percent. The binder composition is curable by the application of heat,i.e., the binder composition is a cross-linked thermoplasticcomposition.

The binder composition is an acrylic latex with a glass transitiontemperature, Tg, in the range of −30 to +50 degrees C., more typicallyin the range of about minus 5 to about 40 degrees C. and most typicallyin the range of about 5 to about 30 degrees C., or equivalent performingresins and Tg's. The acrylic latex can be modified by addition ofmelamine formaldehyde to enhance cross-linking which adds stiffness andheat resistance. Suitable acrylic latex materials include Hycar® 26138,a +25 C. T_(g) a material manufactured by Noveon of Brecksville, Ohio.This latex when mixed with about 2.5% melamine-formaldehyde (MF) resinsuch as Aerotex™ 3030 also made by Noveon yields a moderately stiff matthat pleats well and also has the added advantage of requiring noadditional mechanical support to hold the pleats such as metal wire orcomplex cardboard frames for the resulting filters. Another suitableacrylic latex is Hystretch™ V-29, also made by Noveon. This material isa −29 C T_(g) latex that yields a very soft and flexible fabric orapplications not requiring a self supporting pleat. The bindercomposition includes a homopolymer or copolymer of polyacrylic acid.Typically, the average molecular weight of the polyacrylic acid polymeris less than 10,000, more typically less than 5,000, and most typicallyabout 3,000 or less, with about 2000 being especially good. Use of a lowmolecular weight polyacrylic acid polymer in a low-pH binder compositioncan result in a final product which exhibits excellent structuralrecovery and rigidity characteristics. The binder composition can alsoinclude at least one additional polycarboxy polymer such as, forexample, a polycarboxy polymer disclosed in U.S. Pat. No. 6,331,350, theentire contents of which are incorporated by reference herein.

The glass and polyester fibers that form the base material can be formedinto a structure suitable for use as an air filter. Any suitable meansfor forming the fibers into a mat can be used. For example, the fiberscan be formed by the processes described in U.S. Pat. Nos. 5,840,413,5,772,846, 4,112,174, 4,681,802 and 4,810,576, the entire contents ofwhich are incorporated by reference herein. A dilute aqueous slurry ofthe glass and polymer fibers can be formed and deposited onto aninclined moving screen forming wire to dewater the slurry and form a wetnonwoven fibrous mat in a conventional manner. For example, aHydroformer available from Voith-Sulzer located in Appleton, Wis., or aDeltaformer available from Valmet/Sandy Hill located in Glenns Falls,N.Y., can be used. Other similar wet mat machines can be used.

After forming the wet, uncured web, it is preferably transferred to asecond moving screen running through a binder application station wherethe aqueous binder described above is applied to the mat. The binder canbe applied to the structure by any suitable means including, forexample, air or airless spraying, padding, saturating, roll coating,curtain coating, beater deposition, coagulation or dip and squeezeapplication. The excess binder, if present, is removed to produce thedesired binder level in the mat. The web is formed and the binder levelcontrolled to produce a binder content in the finished dry mat asdescribed-above and to produce a dry mat product having a basis weightof between about 0.5 lbs./100 sq. ft. to about 3 lbs./100 sq. ft., moretypically from about 0.75 lbs./100 sq. ft. to about 1.75 lbs./100 sq.ft. such as about 1.25±0.25 lbs./100 sq. ft. The wet mat is thenpreferably transferred to a moving oven belt which transports the wetmat through a drying and curing oven such as, for example, a throughair, air float or air impingement oven. Prior to curing, the wet mat canbe optionally slightly compressed, if desired, to give the finishedproduct a predetermined thickness and surface finish.

In the oven, the bindered web can be heated to effect drying and/orcuring forming a dry mat bonded with a cured binder. For example, heatedair can be passed through the mat to remove the water and cure thebinder. For example, the heat treatment can be around 400 F. or higher,but preferably the mat is at or near the hot air temperature for only afew seconds in the downstream end portion of the oven. The duration ofthe heat treatment can be any suitable period of time such as, forexample, from about 3 seconds to 5 minutes or more, but normally takesless than 3 minutes, preferably less than 2 minutes and most preferablyless than 1 minute. It is within the ordinary skill of the art, giventhe this disclosure, to vary the curing conditions to optimize or modifythe mat to have the desired properties. The drying and curing functionscan be conducted in two or more distinct steps. For example, the bindercomposition can be first heated at a temperature and for a timesufficient to substantially dry but not to substantially cure thecomposition and then heated for a second time at a higher temperatureand/or for a longer period of time to effect curing. Such a procedure,referred to as “B-staging,” can be used to provide binder-treatednonwoven, for example, in roll form, which can at a later stage becured, with or without forming or molding into a particularconfiguration, concurrent with the curing process. The followingexamples are provided for illustrative purposes and are in no wayintended to limit the scope of the present invention.

EXAMPLE 1

A self supporting MERV 6 media was made using a wet process describedabove with 40% H 117 glass fibers from Johns Manville of Denver, Colo.,40 wt. % 1.5 d PET fibers and 20 wt. % of a binder consisting of 97.5wt. percent Hycar™ 26138 acrylic latex and 2.5 wt. percent Aerotex 3030,both from Noveon. The media was converted into a pleated self supportingMERV 6 filter panel in a conventional manner. The resulting propertiesare shown in Table 1.

EXAMPLE 2

A MERV 6 media for a wire supported filter panel was made using a wetprocess described above with 40 wt. % H117 glass fibers, 40 wt. % 1.5 dPET fibers and 20 wt. % of a binder consisting of 100% Hystretch™ V-29acrylic latex from Noveon. The media was converted into a pleated wiresupported MERV 6 filter panel in a conventional manner. The resultingproperties are shown in Table 1.

EXAMPLE 3

A self supporting MERV 7 media was made using a wet process describedabove with 36 wt. % H117 glass, 32 wt. % 1.5 d PET fiber, 12 wt. %cellulose fluff pulp, and 20 wt. % of a binder consisting of 97.5%Hycar® 26138 acrylic latex and Aerotex™ 3030, both from Noveon. Themedia was converted into a pleated self supporting MERV 7 filter panelin a conventional manner. The resulting properties are shown in Table 1.

EXAMPLE 4

A self supporting MERV 7 media was made using a wet process describedabove with 36 wt. % H117 glass fibers, 36 wt. % 1.5 d PET fiber, 8 wt. %3 micron mean diameter glass microfiber, and 20 wt. % of a binderconsisting of 97.5% Hycar® 26138 acrylic latex and 2.5 wt. % Aerotex™3030, both from Noveon. The media was converted into a pleated selfsupporting MERV 7 filter panel. The resulting properties are shown inTable 1.

TABLE I Data From Examples Ex. 1 Ex. 2 Ex. 3 Ex. 4 Basis wt. gms/sq. m57.15 61.55 59 63 Thickness (mils) 26 23 24 27 Binder Tg 25 −29 25 25 MDTensile (lbs./3 in.) 45 16 55 52 CMD Tensile (lbs./3 in.) 53 20 59 56Tabor Stiffness 14.5 2.7 16.8 15.3 Fraser Air Perm (CFM/sq. ft.)* 643546 531 489 Flat Sheet Eff. (%)** 6 6.7 11 13.8 Full Filter Eff.*** (%)36 NA 44 NA Full Filter Delta P (in. WC 0.101 NA 0.161 NA *At 0.5 in.water column (WC) **A 2 ft. by 2 ft. sample of mat was tested at an airflow rate of 21.6 CFM with same particles as the standard full filtertest. ***After mat has been pleated and assembled into a filter it wastested using the ASHRAE 52.2 Standard Test Procedure.

The filters made with the mat of Examples 1 and 3 met MERV 6specifications and it is expected that the mat of Example 4, whenassembled into a filter, will also meet MERV 6 specifications andpossibly a higher MERV specification. Example 2 shows that the Taborstiffness of the mat should be at least about 2, more typically at leastabout 5, and Examples 1, 3 and 4 show that the Tabor stiffness is mosttypically at least about 10.

By modifying the above method in the drying/curing step and using athermosetting binder such as phenol-formaldehyde, melamine-formaldehydeor polyacrylic acid crosslinked with a polyol, a mat with differentcharacteristics is produced. The modification is to drop the temperaturein the oven such that the binder in the mat is cured to only a “B” stagecondition. This can be achieved by heating the mat to only about 250degrees F. in the oven. The time at lower maximum temperature can bevaried, but typical time is about 30 seconds. Mats made with thismodification can be thermoformed to a desired shape, or pleated and thenheated to complete the cure of the binder. The desired shape will thenbe retained in the mat. Such molded shapes can have many uses such asperforms for SRIM and laminating processes, pleated filters and manyother uses.

While the invention has been described with preferred embodiments, it isto be understood that variations and modifications can be resorted to aswill be apparent to those skilled in the art. Just for the purposes ofillustration of variations included in the present invention, carbonblack can be incorporated into the binder to affect color as can titaniaparticles if a white mat is desired.

1. A method for making a fibrous nonwoven filter media comprising; a)dispersing fibers comprising two different types of fibers in a fluiddispersion, b) subjecting the dispersion to a moving forming screen toform a fibrous web, c) applying an aqueous resin binder to the web, andd) drying the wet web and at least partially curing the resin in thebinder to form a resin bound fibrous non woven mat, wherein; i) thefiber dispersion comprises about 15 to about 60 weight percent polymerfibers and about 60 to about 20 weight percent glass fibers, based onthe total weight of the fibers in the dispersion, and ii) the aqueousbinder comprises a mixture of water and a resin having a glasstransition temperature in the range of about minus 30 to about 50degrees C. whereby the resultant filter media, when pleated and formedinto a pleated gaseous filter using conventional pleating and assemblytechniques produces a filter meeting at least MERV 6 specifications andrequires no additional means of support in the pleated gaseous filter.2. The method according to claim 1, wherein the binder is present in anamount of about 15 to about 40 wt. percent of the dry mat.
 3. The methodaccording to claim 1 wherein the binder is a melamine formaldehydemodified acrylic latex.
 4. The method according to claim 2 wherein thebinder is a melamine formaldehyde modified acrylic latex.
 5. The methodof claim 1 wherein the binder content is in the range of about 15 toabout 30 wt. percent.
 6. The method of claim 3 wherein the bindercontent is in the range of about 15 to about 30 wt. percent.
 7. Themethod of claim 1 wherein the binder content is in the range of about 18to about 25 wt. percent.
 8. The method of claim 3 wherein the bindercontent is in the range of about 18 to about 25 wt. percent.
 9. Themethod of claim 1 wherein the Tg of the binder is in the range of aboutminus 5 to about 40 degrees C.
 10. The method of claim 3 wherein the Tgof the binder is in the range of about minus 5 to about 40 degrees C.11. The method of claim 8 wherein the Tg of the binder is in the rangeof about minus 5 to about 40 degrees C.
 12. The method of claim 1wherein the Tg of the binder is in the range of about 5 to about 30degrees C.
 13. The method of claim 3 wherein the Tg of the binder is inthe range of about 5 to about 30 degrees C.
 14. The method of claim 8wherein the Tg of the binder is in the range of about 5 to about 30degrees C.
 15. A wet laid fibrous nonwoven filter media comprising ablend of fibers comprising about 20 to about 60 weight percent glassfibers and about 15 to about 60 percent man-made polymer fibers in anonwoven mat, the fibers in the mat being bound together by about 15 toabout 40 wt. percent of a binder having a Tg in the range of about minus30 to about 50 degrees C., the filter media, after pleating in aconventional manner and assembled into a gaseous filter usingconventional techniques meets at least MERV 6 specifications.
 16. Thefilter media of claim 15 wherein the binder is present in an amount ofabout 15 to about 40 wt. percent of the dry mat.
 17. The filter media ofclaim 15 wherein the binder is a melamine formaldehyde modified acryliclatex.
 18. The filter media of claim 16 wherein the binder is a melamineformaldehyde modified acrylic latex.
 19. The filter media of claim 15wherein the binder content is in the range of about 15 to about 30 wt.percent.
 20. The filter media of claim 17 wherein the binder content isin the range of about 15 to about 30 wt. percent.
 21. The filter mediaof claim 15 wherein the binder content is in the range of about 18 toabout 25 wt. percent.
 22. The filter media of claim 17 wherein thebinder content is in the range of about 18 to about 25 wt. percent. 23.The filter media of claim 15 wherein the Tg of the binder is in therange of about minus 5 to about 40 degrees C.
 24. The filter media ofclaim 17 wherein the Tg of the binder is in the range of about minus 5to about 40 degrees C.
 25. The filter media of claim 22 wherein the Tgof the binder is in the range of about minus 5 to about 40 degrees C.26. The filter media of claim 15 wherein the Tg of the binder is in therange of about 5 to about 30 degrees C.
 27. The filter media of claim 17wherein the Tg of the binder is in the range of about 5 to about 30degrees C.
 28. The filter media of claim 22 wherein the Tg of the binderis in the range of about 5 to about 30 degrees C.
 29. A filter for gaseshaving a MERV rating of 6, the filter media consisting essentially of awet laid filter media, the wet laid filter media comprising a blend offibers comprising about 20 to about 60 weight percent glass fibers andabout 15 to about 60 percent man-made polymer fibers in a nonwovenpleated mat, the fibers in the mat being bound together by about 15 toabout 40 wt. percent of a binder.
 30. The filter of claim 30 wherein thebinder has a Tg in the range of about minus 30 to about 50 degrees C.31. The filter of claim 30 wherein the binder has a Tg in the range ofabout minus 5 to about 40 degrees C.
 32. The filter of claim 30 whereinthe wet laid mat has a Tabor stiffness of at least
 2. 33. The filter ofclaim 30 wherein the wet laid mat has a Tabor stiffness of at least 5.34. The filter of claim 30 wherein the wet laid mat has a Taborstiffness of at least 10.